Over 1.5 million sport-related mild traumatic brain injuries (mTBI) occur annually in the US, including 25,000 in high school athletes alone. Increased media and medical attention is focusing on these injuries and their potential to cause long-term cognitive, somatic and affective problems. While detection of the low level diffuse damage incurred through mTBI needs to take place accurately and quickly, assessment methods have been criticized as insufficiently sensitive and susceptible to motivational and other extraneous factors. Recent research suggests that oculomotor performance (e.g., eye movements such as saccades and smooth pursuit) may represent a sensitive biomarker of mTBI, yet it has yet to be tested in the real world athletic setting. The first aim of this study i to build and validate a sideline eye tracker that would be portable, low cost, and usable in direct sunlight. Using the sideline eye tracker, we will then evaluate the clinical utility of oculomotor performance as a rapid, objective, and accurate sidelines measure of mTBI among female and male high school and college level athletes. Specifically, we propose to conduct a double case-controlled, longitudinal comparison of 60 concussed athletes, 60 matched non-concussed athlete controls and 60 matched non-athlete controls over two seasons of athletic play, examining all three groups on oculomotor performance and a widely used mTBI diagnostic neuropsychological measure (the ImPACTTM test). The two athlete groups will be tested at baseline (pre-injury), post-injury, and post-season delay. The non-athlete control group will be tested at baseline and post-season delay. We will investigate the test-retest reliability of the oculomotor performance variables, document any change from baseline following mTBI, explore how oculomotor performance relates to cognitive performance (ImPACT test) and differs between the three subject groups, and examine whether change from baseline predicts recovery rate (days till return to play). Finally, we will conduct a preliminary examination of whether a multi-variate statistical model of oculomotor performance can classify subjects into concussed and non-concussed groups.